Thin plastic sheets are often utilized in the automotive industry to form an outer skin or shell covering on automotive products such as door panels, instrument panels or the like. The sheet is typically shaped by vacuum molding whereby the sheet is positioned over a mold member defining a mold cavity, and a vacuum is applied so that the sheet is sucked into the mold. Prior to vacuum molding of the sheet, the sheet must typically be heated to a certain temperature in order to provide flexibility to the sheet to enable proper shaping thereof.
One conventional heating method and apparatus employs quartz heaters oriented above and below the plastic sheet as the sheet is moved through the heating unit towards the forming unit by means of a conveyor arrangement. The conveyor arrangement includes a conveyor member or chain which grips and supports the sheet on opposite sides thereof. However, the increased usage of vinyl sheets having a ultraviolet (UV) resistant constituent has created problems in conventional heating methods and units, in that when the sheet is heated to the necessary forming temperature, the sheet undergoes excessive sagging or elongation which causes undesired thinning and can result in a nonuniform sheet. Also, this can result in the heated sheet contacting the heaters or other structural elements which thus damages or blemishes the sheet. One attempt to solve this problem included positioning the heaters farther apart from one another and thus farther from the sheet. However, this solution necessarily required increased energy and process time in order to heat the sheet to the required temperature, which is disadvantageous from a manufacturing standpoint.
It is an object of the present invention to provide an improved apparatus and method for heating a thin, elongate sheet of material, such as plastic, and more specifically vinyl containing a UV resistant constituent. The improved apparatus and method are believed to significantly improve the overall performance characteristics of the apparatus and process in terms of efficiency by providing a more effective method of supporting the sheet during the heating thereof prior to the forming process, thus avoiding excessive sheet deformity due to sagging or stretching which can result in an inferior or damaged sheet product, while permitting efficient heating of the sheet.
The improved apparatus and process according to the present invention includes a heating unit having a housing with a tunnel-like configuration which defines an entrance end through which the sheet enters as it is moved along by a conventional conveyor unit, and an exit end through which the sheet exits after the heating thereof. The heating unit includes a support arrangement which supports the sheet in a generally horizontal manner during its travel through the heating unit, which support arrangement includes guide members arranged at opposite longitudinal sides of the sheet. Each guide member serves to guide an endless conveyor member or chain of the conveyor unit, which conveyor members grip and carry the sheet along the opposite longitudinal edges thereof. Heating elements are disposed above and below the sheet along the length of the heating unit, and a diffuser plate is arranged between the downwardly facing side of the sheet and the lower heating elements. The diffuser plate permits air flow upwardly therethrough as supplied from an air supply system, and thus enables the formation of a film of air under the sheet to support same and prevent excessive sagging or transverse elongation of the heated sheet during transport through the heating unit.
The advantageous arrangement and method of the present invention, and objects and purposes thereof, will be apparent to persons familiar with apparatus and methods of this general type upon reading the following specification and inspecting the accompanying drawings.